Apparatus, system, and method for wavelength conversion of mode-locked extended cavity surface emitting semiconductor lasers

Abstract

A mode-locked laser with intracavity frequency conversion is disclosed. In one embodiment the conversion frequency is improved by reducing the temporal, spatial, or polarization overlap between pulses at the fundamental frequency and pulses at a frequency-shifted frequency.

1 . An apparatus for improving the efficiency of a mode-locked laser having multi-pass optical frequency conversion, comprising: 
 a time delay element for delaying the transmission of optical pulses;    an entrance to said time delay element configured to be transmissive to optical pulses at a fundamental frequency of said mode-locked laser and reflective at a second frequency; and    an end reflector for reflecting light at said fundamental frequency back towards said entrance;    wherein said time delay element introduces a time delay between optical pulses at said fundamental frequency emerging from said entrance and optical pulses at said second frequency reflected from said entrance whereby interference effects in a subsequent nonlinear optical material used to generate light at said second frequency are reduced.    
     
     
         2 . The apparatus of  claim 1 , further comprising: an optical gain region for generating gain at said fundamental frequency.  
     
     
         3 . The apparatus of  claim 1 , wherein said time delay element comprises a length of semiconductor material have a low optical loss for light at said fundamental frequency and introduces a time delay corresponding to at least a portion of the spectral bandwidth of optical pulses in said mode-locked laser whereby optical pulses at said fundamental frequency are at least partially temporally separated from optical pulses at said second frequency.  
     
     
         4 . The apparatus of  claim 1 , wherein said entrance includes an optical coating that is transmissive to said fundamental frequency and reflective to said second frequency.  
     
     
         5 . The apparatus of  claim 1 , further comprising a lens selected such that reflected light at said second frequency is defocused with respect to light emerging from said entrance at said fundamental frequency.  
     
     
         6 . The apparatus of  claim 1 , wherein said mode-locked laser modulator comprises a saturable quantum well absorber.  
     
     
         7 . The apparatus of  claim 1 , wherein said end reflector comprises a Bragg reflector formed in a semiconductor element.  
     
     
         8 . The apparatus of  claim 1 , further comprising a mode-locked laser modulator.  
     
     
         9 . A semiconductor element for improving the efficiency of a mode-locked laser having multi-pass intra-cavity frequency doubling, comprising: 
 a time delay element formed from a first region of said semiconductor element for delaying the transmission of optical pulses;    an optical coating formed on a front surface of said semiconductor element configured to be transmissive to optical pulses at a fundamental frequency of said mode-locked laser and reflective at a harmonic frequency;    a quantum well saturable absorber formed in a second region of said semiconductor element;    an end reflector formed in a third region of said semiconductor element for reflecting light at said fundamental frequency back towards said optical entrance;    wherein said time delay element introduces a time delay between optical pulses at said fundamental frequency and optical pulses at said harmonic frequency directed towards a nonlinear optical material whereby interference effects in said nonlinear optical material are reduced.    
     
     
         10 . The apparatus of  claim 9 , further comprising: an optical gain region disposed in a fourth region of said semiconductor element for generating gain at said fundamental frequency.  
     
     
         11 . The apparatus of  claim 9 , further comprising a lens formed in said semiconductor element such that reflected light at said harmonic frequency is defocused with respect to light emerging from said entrance at said fundamental frequency.  
     
     
         12 . A method of operating a mode-locked laser, comprising: 
 providing a nonlinear material within an optical resonator for frequency conversion of optical pulses at a fundamental frequency;    generating mode-locked laser pulses at said fundamental frequency within said optical resonator;    in a first pass through said nonlinear material, generating an optical pulse at a harmonic frequency to form a first pulse at a harmonic frequency;    time delaying a partially depleted optical pulse at said fundamental frequency output received from said nonlinear material to generate a time delayed fundamental pulse; and    coupling said first pulse at said harmonic frequency and said time delayed fundamental pulse back to said nonlinear material to generate a second pulse at said harmonic frequency.    
     
     
         13 . A mode-locked laser, comprising: 
 an optical resonator;    a laser gain element disposed in said optical resonator for providing optical gain about a fundamental laser frequency;    a mode-locking modulator disposed in said optical resonator;    a nonlinear optical material disposed in said optical resonator for performing optical frequency conversion in which an input pulse at said fundamental laser frequency is converted into an output pulse of reduced power at said fundamental laser frequency and an output optical pulse at a harmonic frequency; and    a frequency selective time delay element disposed in said optical resonator, said frequency selective time delay element introducing a time delay between optical pulses at said fundamental laser frequency and optical pulses at said second harmonic wavelength whereby interference between optical pulses at said harmonic frequency and said fundamental frequency in said nonlinear optical material is reduced.    
     
     
         14 . The laser of  claim 13 , wherein said mode-locking modulator comprises a saturable absorber.  
     
     
         15 . The laser of  claim 14  where the saturable absorber comprises quantum wells selected from the group of materials consisting of GaInAs, GaAsP, GaAlAs, and GaInAsP.  
     
     
         16 . The laser of  claim 14  in which said saturable absorber is grown adjacent to a highly reflective semiconductor Bragg mirror made up of alternate layers of GaAlAs and GaAs that serve as one of the mirrors in the laser resonator.  
     
     
         17 . The laser of  claim 13  in which the laser gain element is selected from the group of semiconductor laser materials consisting of GaAlAs, GaInAs, GaAsP, and GaInAsP.  
     
     
         18 . The laser of  claim 13  in which the saturable absorber is grown on a semiconductor substrate adjacent to a gain media forming said gain element.  
     
     
         19 . The laser of  claim 13  in which said saturable absorber comprises at least one quantum well disposed in a p-n semiconductor junction for applying a reverse bias voltage to said at least one quantum well to adjust optical loss of said saturable absorber.  
     
     
         20 . The laser of  claim 13  in which the voltage applied to the saturable absorber can turn the laser off and on to produce a modulated train of mode-locked pulses at the fundamental and the second harmonic.  
     
     
         21 . The laser of  claim 19  in which the current of said saturable absorber is used to monitor the laser power.  
     
     
         22 . The laser of  claim 13  in which the laser gain medium is selected from the group consisting of solid-state, gas, semiconductor, and liquid laser medium.  
     
     
         23 . The laser of  claim 13  in which the nonlinear material is selected from the group consisting of poled lithium niobate, poled KTP, poled lithium tantalate, poled potassium niobate, un-poled bulk lithium niobate, unpoled bulk BBO, unpoled LBO, and unpoled KTP.  
     
     
         24 . The laser of  claim 13  in which the nonlinear conversion is selected from the group of frequency conversion processes consisting of frequency doubling, frequency tripling, frequency quadrupling, and wavelength down-conversion.  
     
     
         25 . The laser of  claim 13  in which there are a multiple of devices arranged in a one- or two-dimensional array in which the devices are independently addressable.  
     
     
         26 . An extended cavity semiconductor laser, comprising: 
 a surface emitting semiconductor element including: 
 a quantum well gain region; and  
 an integrated quantum well saturable absorber for providing mode-locking;  
 at least one Bragg reflector; and  
   an external mirror.    
     
     
         27 . A mode-locked laser, comprising: 
 an optical resonator;    a laser gain element disposed in said optical resonator for providing optical gain about a fundamental laser frequency;    a mode-locking modulator disposed in said optical resonator;    a nonlinear optical material disposed in said optical resonator for performing optical frequency conversion in which an input pulse at said fundamental laser frequency is converted into an output pulse of reduced power at said fundamental laser frequency and an output optical pulse at a harmonic frequency; and    an element disposed in said optical resonator configured to at least partially reduce the spatial, temporal, or polarization overlap of output optical pulses at said harmonic frequency with optical pulses at said harmonic frequency whereby interference between optical pulses at said harmonic frequency and said fundamental frequency in said nonlinear optical material are reduced.    
     
     
         28 . A method of operating a mode-locked laser, comprising: 
 providing a nonlinear optical material within an optical resonator for frequency conversion of optical pulses at a fundamental frequency;    generating mode-locked laser pulses at said fundamental frequency within said optical resonator;    in a first pass through said nonlinear optical material, generating an optical pulse at a harmonic frequency to form a first pulse at a harmonic frequency and a second optical pulse at said fundamental frequency; and    at least partially reducing a temporal, spatial, or polarization overlap of said first pulse and said second pulse prior to coupling said first pulse and said second pulse back to said nonlinear optical material, whereby interference effects are reduced in said nonlinear optical material.